An active matrix type flat display device being the main current at present is a flat display comprising pixel driving switches composed of TFT (Thin Film Transistor) and a pixel display medium on the surface of it, and a substrate of its starting point is a transparent glass plate of soda lime and the like. Attempts to use a plastic film as a substrate have been performed but do not yet succeed in being put to practical use, and the main current at present is liquid crystal as a display medium and a-Si TFT (Amorphous-Silicon-TFT) as an active matrix, and displays of 10″ to 20″ in diagonal size are mass-produced for PCs, monitors and the like.
LCD (Liquid Crystal Display) as a display medium has problems in display performance of TV dynamic images, particularly in white, white peak, responsiveness and the like in comparison with CRT (Cathode Ray Tube). On the contrary, organic LED (OLED: Organic Light-Emitting-Diode) of which product development is recently promoted is self-luminous and can realize an image quality being more excellent in white, white peak, responsiveness and the like than LCD.
On the other hand, in TFT also, product development of polycrystalline Si by a low-temperature process (low temperature p-Si) is rapidly promoted. The reason is that first, p-Si is high in TFT performance and can have a peripheral circuit built in it and therefore has an advantage of cost reduction. In addition to this, since a-Si TFT is difficult to cope with drive of organic LED from the viewpoint of a driving current density, the transition of TFT to low-temperature p-Si including application of it to LCD is the general trend.
Market demands in all display devices including an active matrix type flat display device are always three points of the enlargement in size, high definition and cost reduction of display device. For these demands, a-Si TFT-LCD being the main current at present has little room for improvement in performance and is in the state that a substantial limit is TV of 40″ in diagonal length for enlargement and display of 20″ or less for high definition and the enlargement of glass substrate may be said to be only one means for cost reduction.
On the other hand, low-temperature p-Si TFT-LCD is excellent in performance of TFT itself in principle and can have a peripheral circuit built in it but has grave problems in practice. That is to say, there are various fundamental problems of processes being performed at a low temperature of 500° C. or lower, lack of uniformity due to polycrystal, lithography accuracy of 1 μm or more and the like because of a glass substrate. Particularly, the low-temperature p-Si TFT must realize a performance equivalent to Si LSI in a peripheral circuit but is difficult to realize a high image quality under such restrictions, and is in the state of being applied to some of peripheral circuits of a low-definition display.
In case that a display medium is organic LED, namely, TFT-OLED (TFT-Organic Liquid-Emitting-Diode), the quality of display is greatly improved in comparison with LCD from the viewpoint of self-luminance, high-speed response, thinning and the like. However, since a pixel driving circuit is composed of several transistors instead of one transistor because of current drive differently from LCD. Although there is the attempt to form a pixel driving circuit out of a-Si TFT due to the advantage of uniformity, a large-sized high-definition display cannot but use p-Si TFT.
However, a substrate is of glass and has fundamental problems with respect to TFT itself as described above, and further a large-sized glass substrate cannot but be used for cost reduction similarly to an a-Si-TFT-LCD fabricating technology.
Organic LED is composed of an organic thin film of 5 to 8 layers, the total film thickness of it is about 100 to 500 nm, and the thickness of each component film must be formed with accuracy of about 1 nm. Additionally, pixels corresponding to three colors must be formed separately over a large area. Further, since current consumption is greatly increased to 10 to 100 mA/cm2 due to current drive in case of OLED while current consumption is 1 μA/cm2 due to voltage drive in case of LCD, wiring resistance from a current source needs to be reduced by several digits in comparison with wiring of LCD. It is apparent that the more large-sized and more high-definition a display is, the more difficult the solution of such problems in manufacture is.
These essential problems are in a fact that they are apparently natural, that is, in a fundamental assumption of the prior art using a two-dimensional flat plate as a substrate. That is to say, the improvement in process accuracy is demanded simultaneously with enlargement in size of a substrate and a fabricating apparatus must be made more accurate at the same time as being made large-sized. Naturally there is some limit in mechanism and a limit appears in throughput also. In practice, an a-Si TFT-LED fabricating apparatus coping with a substrate of nearly 2 m square in size is made and used at present, but this is thought to be one limit with respect to cost-performance ratio of apparatus and production line.
The situation in p-Si TFT-LCD based on the existing a-Si TFT-LCD fabricating apparatus and technology is entirely the same. Additionally, p-Si TFT-LCD is in a more difficult situation that a process similar to that of Si LSI must be realized at a low temperature. Cost reduction by having a circuit built-in is one of advantages of p-Si TFT-LCD, but this is realized when a high-performance circuit is implemented. In practice, the more large-sized a substrate is made, the more difficult it is to realize various requirements necessary for a high-performance device such as the quality of film, the accuracy of photolithography, similar processes to those of Si LSI and the like. As for this point, p-Si TFT-OLED is entirely the same and further additionally has problems of LED structure, wiring resistance and the like as described above.